Minimizing unnecessary edge loops
Getting rid of unnecessary loopcuts is always a good idea, no matter for which platform you aim (mobile devices, PC?). If you can reduce the polycount significantly by using submeshes that's the way you want to go.

When using overlapping submeshes you also might want to delete hidden faces.  

One other thing that is even more important than reducing a model's polycount by maybe a dozen polygons is the use of materials. If you have an asset that contains several submeshes you still want to use the lowest possible amount of materials (which is normally one). Having only one material per asset reduces the draw calls in a scene significantly. You might want to look up the terms "draw call" and "texture atlas". 

Lightning errors in UE4 are mostly the result of missing or wrong lightmaps.